Field of the Invention
The present disclosure relates to an organic light emitting display enabled to improve image quality based on a result of sensing driving characteristic variations of pixels.
Discussion of the Related Art
An active-matrix type organic light emitting display includes Organic Light Emitting Diodes (OLEDs), and it shows a fast reaction speed while its light-emitting efficiency, luminance, and field of view are great. An OLED includes an organic compound layer formed between an anode and a cathode. The organic compound layer includes a Hole Injection Layer (HIL), a Hole Transport Layer (HTL), an Emission Layer (EML), an Electron Transport Layer (ETL), and an Electron Injection Layer (EIL). If a driving voltage is applied to the anode and the cathode, a hole passing through the HTL and an electrode passing through the ETL move to the EML to form an exciton, and thereby, the EML generates a visible light.
Each pixel of the organic light emitting display includes a driving device that controls a current flowing in the OLED. The driving device may be implemented as a Thin Film Transistor (TFT). It is desirable to design the driving device has uniform electrical characteristics, such as a threshold voltage and mobility, in all pixels. However, due to the manufacturing conditions and driving environment, it is hard for the driving TFT to have the uniform electrical characteristics. As time goes by, more stress is applied to the driving device, and the stress may be different depending on a data voltage. The electrical characteristics of the driving device are affected by the stress. Thus, electrical characteristics of the driving TFT are changed once a driving period of time has elapsed.
Methods of compensating for a change in driving characteristics of a pixel in an OLED display device are divided into an inner compensation method and an external compensation method.
The inner compensation method is implemented in a manner of automatically compensating for threshold voltage deviation between driving TFTs in a pixel circuit. For the inner compensation, a current flowing in the OLED needs to be determined regardless of the threshold voltage of the driving TFT, such that a structure of the pixel circuit becomes complex. The inner compensation method is hard to compensate for mobility deviation between the driving TFTs.
The external compensation method is implemented by sensing electrical characteristics (a threshold voltage, mobility, etc.) of the driving TFTs and then modulating pixel data of an input image in a compensation circuit located outside a display panel based on the sensing result so as to compensate for driving characteristic changes of each pixel.
The external compensation method is implemented by receiving a sensing voltage directly from each pixel through a reference voltage line connected to pixels of the display panel, generating a sensing value by converting the sensing voltage into digital sensing data, and then transmitting the sensing value to a timing controller. The timing controller modulates digital video data of an input image based on the sensing value to compensate for driving characteristic changes in each pixel.
As resolution of an organic light emitting displays and efficiency of an organic compound have improved, an amount of a current required to drive a pixel (or a required current for each pixel) has been dramatically reduced. To sense driving characteristic changes of a pixel, a sensing current received from the pixel is also reduced. If the sensing current is reduced, a capacitor of a sample & holder is charged less in a limited sensing period, thereby making it difficult to sense driving characteristic changes of the pixel. The sample & holder charges the sensing current in the capacitor to sample a sensing voltage received from the pixel.
If the sensing current becomes low, it fails to satisfy the minimum resolution of an analog-to-digital converter (ADC) and thus driving characteristics of the pixel cannot be sensed. Basically, the sensing voltage received from the pixel is converted by the ADC into digital data. However, if a current of the pixel becomes low, the sensing voltage received from the pixel becomes lower than the minimum input voltage to the ADC. When driving characteristics of the pixel in low gray-scale data are sensed, a current of the pixel becomes low and thus the driving characteristics of the pixel in a low gray-scale cannot be compensated. On the other hand, a pixel has a great amount of current in high gray-scale data, so that it is possible to sense driving characteristics of a high-resolution and high-contrast pixel.